Add checkbox to toggle visibility of password field, change field input ...

[madmuffin1]

...types, focus first field on activity startup

We are evaluating several Caldav providers and I had the issue that my keyboard (Swiftkey) tried to autocorrect the input of the userfield, so I quickly changed the type of that field to prevent this from happening. Also added a Show Password checkbox, which is very useful for people with super long passwords.